Our long term goal is to understand information processing in visual cortex. A fundamental aspect of cortical processing is the integration of inputs from different sources by single cells and local networks. Here we will focus on the integration of inputs arising from visual activation of areas inside and outside the receptive field (RF). Surround stimuli (outside the RF) facilitate or suppress the responses of visual cortex neurons to stimuli inside their RF in a contrast and orientation dependent manner. It has been proposed that such modulatory effects depend on asymmetries in the response to stimulus contrast between regular spiking (RS) and fast spiking (FS) cells, which are the main excitatory and inhibitory neuronal cortical types, respectively. It is known that FS cells in vitro show firing rates in response to current injection that increase more steeply and reach higher and more sustained values than RS cells. In addition, little is known about the visual properties of the different electrophysiological cell classes of the neocortex. We will use intracellular recordings in vivo in anesthetized animals in concert with both standard and more innovative paradigms of visual stimulation. We will proceed along the following aims: (i) Establish the correspondence between electrophysiological and functional cell properties in the visual cortex, (ii) demonstrate asymmetries in the response to current injection and stimulus contrast between RS and FS cells, and (iii) demonstrate that the changes in surround effects due to contrast are based on a shift on the balance of synaptic inputs from excitatory to inhibitory. The prediction is that this shift will occur only in RS cells, while FS cells will show further facilitation with increases in contrast. This prediction is in complete agreement with our preliminary data and with in vitro studies showing IPSPs in response only to higher intensities of stimulation of the long range cortical connections. In order to characterize the synaptic events that underlie the modulatory actions of surround stimuli, we propose to use flashed bars with different contrasts and orientations. These bars will be flashed at different locations outside the RF and at different times with respect to the bars flashed inside the RF. This experimental approach will also allow us to study the spatiotemporal properties of the modulatory effects of surround stimuli.